Wikipedia:Reference desk/Archives/Science/2017 October 22

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October 22

How does a battery powered pendulum work?

How does a battery operated clock with a pendulum keep the pendulum swinging? I'd like to make something that will keep swinging but keep the mechanism small so it mustn't have any of that nonsense pertaining to time keeping. 185.81.136.19 (talk) 09:16, 22 October 2017 (UTC)

An Escapement is the way a pendulum is kept swinging to compensate for slowing due to friction. DMacks (talk) 09:28, 22 October 2017 (UTC)

That's the mechanical method, but are not electromagnetic pulses more common in battery clocks? (Reference needed.) Dbfirs 09:50, 22 October 2017 (UTC)

• There are (at least) two ways:

The simplest and cheapest way for a clock is to have a battery powered clock mechanism, with a pulse motor driven by a quartz clock circuit. This ticks once per second (or twice that). A pendulum is attached and is 'pumped' by the motor, via the clock mechanism. The Q of the pendulum system is very low, so the system is able to run at a frequency far lower than the natural frequency of the pendulum, as the purely decorative pendulum is often far too short. The pendulum plays no part in the timekeeping.

An older version of this was used in cheap spring-wound cuckoo clocks, which used a balance wheel escapement to control timing, but also had a decorative pendulum. Those often needed to have the pendulum manually adjusted to be approximately in synch, otherwise they could run very unevenly.

A different approach is a pendulum that does the timekeeping (as a pendulum) and is driven by a magnetic arrangement. These are very common today, not as clocks but as solar-powered "dancing flowers" (We ought to have an article on these, they have some neat mechanisms). In these, there are two coils in the base, and a magnet on the pendulum. As the magnet swings past the coil, the "sense" coil detects it (a small current is induced in the coil). This triggers the circuit, which is a simple amplifier and which then puts a pulse through the larger "drive" coil, which accelerates the magnet in the pendulum and kicks it. The feedback and timing is controlled entirely by the mechanical pendulum. Andy Dingley (talk) 11:21, 22 October 2017 (UTC)

Thanks for that excellent explanation, Andy. My battery-powered clock has a quartz oscillator for timekeeping, and a completely independent magnetic pulse circuit to drive the "pretend" pendulum. The clock works with the pendulum stopped. Dbfirs 12:12, 22 October 2017 (UTC)

I did see a clock a while back which was like a big grandfather clock and had a complicated looking pendulum to compensate for the temperature. It operated off a car battery I think and had wires from it so it could drive secondary clocks elsewhere in time with it. I found [1] which describes something very like it. It hhad a display but I'm not altogether certain it was connected to the pendulum, it might have been a slave clock. Dmcq (talk) 15:11, 22 October 2017 (UTC)

Was it a Master clock? There are lots still working near to where I live. Dbfirs 16:59, 22 October 2017 (UTC)

Yes that does describe it well. I had multiple rods in the pendulum rather than the simple looking ones in the article but I guess there must have been quite a number of types. Dmcq (talk) 17:48, 22 October 2017 (UTC)

A master clock is almost the opposite of what we were discussing here. It's an old idea for a mechanical pendulum clock, controlled by the pendulum's mechanical timekeeping, which then operates switch contacts and in turn drives a number of other clocks. The pendulum clock also electrically powers itself, but this is (like the dancing flowers) just a power amplifier, not a timing regulator. The slave clocks are electrically powered and very simple.

These were found in many telephone exchanges, where they would then drive clocks throughout the building and beyond. They often drove tower clocks (clocks in towers) because this made the clock in the exposed position much simpler and more reliable, keeping the sensitive parts closer to hand. For accuracy they usually had long pendulums but only rarely a gridiron pendulum for temperature compensation. It was more usual to have them themselves synchronised to a more accurate master clock (usually Greenwich), and those clocks would have an automatic synchronising mechanism (a "heart-shaped cam") as well. Andy Dingley (talk) 19:24, 22 October 2017 (UTC)

Another option is to just keep the pendulum mechanism entirely unpowered and unused for time-keeping. That is, it can use weights to run the pendulum, but not use the pendulum for anything. Thus, you could have the accuracy of a quartz clock with the "old-timey" feel of a pendulum. You could let the pendulum run down, when nobody is around to impress by it, rather than have it use up batteries. Then set the weights when company is coming over. StuRat (talk) 22:08, 22 October 2017 (UTC)

... only if the guests are making a very short visit! Dbfirs 00:01, 23 October 2017 (UTC)

A low friction pendulum should be able to continue for several hours, at least. See Longcase_clock#Description, which mentions an 8-day mechanism and a 30-hour mechanism. This one is supposedly on the small side, with corresponding reduction in run time, but shouldn't expend any pendulum energy moving the hands or chiming, which should make it last longer. StuRat (talk) 00:05, 23 October 2017 (UTC)

No, that is not an example of an unpowered pendulum. Those pendulums are driven by weights and have escapement mechanisms. I suggest you try an undriven pendulum yourself to see how long it lasts. Apologies if I misunderstood your suggestion. I do like your idea of having an accurate quartz mechanism for timekeeping but retaining the weight-powered pendulum for effect. Dbfirs 07:04, 23 October 2017 (UTC)

I meant unpowered by electricity. Weights are exactly what I had in mind. Sorry if I was unclear. Electricity to drive the quartz clock, and weights to drive the pendulum, and no interaction between the two. StuRat (talk) 03:38, 25 October 2017 (UTC)

In a US school built in the 1930's, which I saw decades later, there was still a self winding clock in the Principal's office. It was a wooden-cased wall-mounted pendulum clock perhaps 24 inches tall, with a large dry-cell battery visible in the bottom of the case. It also said, if memory serves. "Western Union Time" or some such claim. I doubt they actually had a dedicated telegraph line for time setting because the place was very cheaply run, but the clock was made to have the capabiliy for automatic time correction. At some set intervals the battery would rewind it by winding a spring, and it had moveable contacts which would ring the bells for passing periods. That is one way a pendulum clock could be powered by electricity. Scientific American had an Amateur Scientist article (Oscillator for pendulum clock. quartz-crystal, 1974 Sep, pg 192 is probably the article but I lack online access) about how someone gave an antique grandfather clock crystal accuracy. He installed a coil near the steel pendulum, or installed a small magnet on the pendulum. When the pendulum neared to coil, a small voltage could be detected by a transistorized circuit, which then sent out a pulse of electricity to correct any lateness or earliness of the pendulum travel. That clock may have had a descending weight as the main power source but it would be trivial to have a couple of coils provide all the motive force, though the pendulum would be along for the ride in determining accuracy as long as it was not grossly too long or short. Edison (talk) 20:11, 25 October 2017 (UTC)

Suitable conduit for conveyance of steam from kettle

I would like to make a steamer for wood bending and I've read that a PVC pipe (like a drain pipe?) can be used as a vessel for this but I need a conduit to convey steam from my kettle on the gas stove to my PVC pipe. What would be a suitable but cheap material for such a conduit? How hot is it likely to get? I figure no more than 150 °C? --178.170.142.48 (talk) 22:50, 22 October 2017 (UTC)

To clarify, do you mean to convey the steam under pressure, or just at normal atmospheric pressure ? StuRat (talk) 23:01, 22 October 2017 (UTC)

Yes, the pressure would only be slightly higher than atmospheric. 23:27, 22 October 2017 (UTC)

If you design it like this one you don't need any conduit: [2] The steam box sits directly over the steam source. Rmhermen (talk) 23:17, 22 October 2017 (UTC)

Meaning directly over the kettle ? StuRat (talk) 23:22, 22 October 2017 (UTC)

I think that would just shift my problem somewhere else. I'd need to find a way to perch my wood container over the source of the steam in such a way as to capture it efficiently (and I don't wish to make a heater when I can just use my gas stove). --178.170.142.48 (talk) 23:27, 22 October 2017 (UTC)

This one uses a PVC pipe and a garden hose: [3] Rmhermen (talk) 23:21, 22 October 2017 (UTC)

• Plain old PVC should work just fine, because that steam cannot be much hotter than boiling. However, if you want a conservative design, you can use CPVC instead for the conduit. CPVC is normally used instead of PVC for hot water under pressure. If you are truly paranoid, use a copper or stainless flex pipe of the type sold to connect a hot water heater to the house plumbing. -Arch dude (talk) 03:09, 23 October 2017 (UTC)
  • If you find that the steam is losing too much heat and/or condensing in your pipe, hardware stores also sell closed-cell insulation that goes around hot water pipes. --Guy Macon (talk) 03:45, 23 October 2017 (UTC)

• My steam bender is made from a scrap offcut of large diameter plastic underground gas pipe, the yellow plastic stuff. The steam is supplied by one or two wallpaper steamers. These are cheap (as cheap as a kettle), larger water capacity and already have a hose attached. You should avoid steel in making such a steamer - especially if you're working oak or chestnut - as there is a risk of blue-black or rust staining if iron is combined with the tannates inn the wood.

Generally any sort of flexible plastic pipe from a typical plumber or electrician will do. Lots of people use 20mm flexible electrical conduit, which is usually PVC. Andy Dingley (talk) 09:59, 23 October 2017 (UTC)